Frequency synthesizers are widely used in modern portable radios because they provide several stable frequencies without the need of a crystal for each frequency of operation. For example, a known frequency synthesizer for use in a battery-powered two-way radio is shown in FIG. 1. The radio 10 comprises a receiver front end 12, a mixer 14, a first intermediate frequency (IF) stage 16, and a frequency synthesizer circuit. The frequency synthesizer circuit comprises a voltage-controlled oscillator (VCO) 18, a buffer 20, a divide-by-N frequency divider 22, a phase detector 26, a low-pass filter 28, and a buffer 30. A reference oscillator 24 provides a waveform having a reference frequency for mixing with the output signal of the VCO. This synthesizer is used to allow for programmability of the receive and transmit frequencies. One drawback of synthesizers, however, is that they consume more current than crystal-controlled oscillators. In particular, the high speed dividers 22, phase detector 26, and loop filter 28 (enclosed in dashed lines in FIG. 1) can draw significant amount of current (as much as 10 mA for an 800 MHz synthesizer). Moreover, since a portable radio, during typical usage, operates in the receive standby mode for 80-90% of the time, where the usual current drains are in the 40-60 mA range, the divider and phase detector current represent a significant percentage of the total standby current and therefore have an important impact on total battery life.
An example of a battery saver frequency synthesizer arrangement was taught in U.S. Pat. No. 4,521,918 to Challen (1985). Challen teaches replacing a control voltage produced by a PLL synthesizer with an automatic frequency control voltage produced by the discriminator, and interrupting power to the PLL synthesizer, during a battery saver mode. However, the Challen approach is limited to use in a strong RF environment, since it depended on the automatic frequency control (AFC) feedback loop using the discriminator output, which becomes noisy when the RF carrier signal is weak. While the average signal level can be monitored by having a microcontroller read the received signal strength indicator (RSSI) signal from the IF stage, and therefore decide when to use the battery saver scheme, a sudden RF fade could cause unreliable VCO operation for several tens of milliseconds before the microcontroller can react and switch back to standard synthesizer operation. Another drawback of the Challen approach is its requirement that an RF carrier be present at all times so as to be able to generate a valid discriminator signal to control the AFC feedback loop. This requirement limits its usefulness to a trunked system in which the control channel is always active. Thus, a need exists for a frequency synthesizer that operates in a battery saver mode, and avoids the detriments of the prior art.